Fertilizers are responsible for over half of global food production, but there are areas in world with nutrient deficiency and other areas of nutrient excess.
Managing mineral plant nutrients requires careful application of science and skill to meet production, environmental, and social goals.

Monday, February 11, 2013

Meeting the Phosphorus Requirement on Organic Farms

Using organic nutrient sources?

Phosphorus management can be difficult in
organic production since approved sources are limited and the consequences of
under- or over-fertilization can be significant. Since P is an essential
element for plant growth involved in many critical plant metabolic functions,
sustainable agricultural production depends on an adequate P supply.

This article was published in Better Crops by Nathan Nelson and myself several years ago. The original pdf version of the article is available here.

In most agricultural systems…both organic
and conventional… complete nutrient cycling does not occur. The
nutrient reservoir in the soil shrinks when crops are removed from the field at
harvest. This nutrient export creates a P deﬁcit, necessitating regular P
additions to replace the harvested P. Several studies investigating whole-farm
P budgets have found nutrient P deﬁcits in many organic farms and illustrate
the need for nutrient additions. Because P is an essential nutrient for plant
growth, all sustainable systems should at a minimum seek to replace the P
removed in harvested crops in order to avoid declines in yield and quality.
Although organic agriculture seeks to minimize off-farm inputs, it is essential
that producers replace P removed in harvested crops.

Nutrient management is complex in mixed livestock/crop farming.

A brief review of the most commonly used
P sources for organic production is presented here. More information and an
extensive list of references are available at the website:

Soil organic matter can be an important
source of P for crops. Some studies have shown that soil organic matter
increases on organically managed farms, while other long-term studies do not
show such a buildup. These differences largely depend on management practices
such as tillage intensity, heavy manure additions, return of crop residues, the
extent of cover cropping, and climatic factors. Soil organic matter serves as a
reservoir of plant nutrients, but may also improve the soil physical conditions
and root environment.

Soil organic matter contains a variety of
organic P compounds, such as inositol phosphate, nucleic acid, and phospholipid. These compounds must be first converted to inorganic phosphate by
soil enzymes before being used for plant growth. These phosphatase enzymes are
produced by soil microorganisms, mycorrhizal fungi, or excreted by the plant
root. Some organic P compounds are stable for many years in the soil, while
others are converted to inorganic P within a few days or weeks.

Cover Crops

Cover crops can recycle phosphorus

Cover crops are frequently grown in
rotation with cash crops for a variety of beneficial purposes. The advantage of
cover crops for P nutrition involves the accumulation of soil P by the cover
crop. This P is subsequently released when the cover crop is killed. Numerous
studies have shown that some cover crops can provide a P nutritional benefit
for the next crop compared to crops grown without a preceding cover crop. This
is attributed to the ability of some species to draw down soil P concentrations
below what some cash crops can and also to their extensive root system. This P
drawdown may also be the result of root exudates and the efficient P uptake by
the cover crop roots. Some cover crops can be excellent hosts for mycorrhizal
fungi, which may allow a greater exploitation of the soil P reserves.

There are considerable differences in the
ability of various cover crops to provide additional P for the subsequent crop.
Research has generally shown a greater P benefit from legume cover crops than
from grass cover crops, but the effects of cover crops on P nutrition can be
highly variable. In many cases, supplemental P is still required after the
cover crop to eliminate P deficiency. In some circumstances, P uptake by the
cash crop following the cover crop is actually reduced due to low residual soil
P caused by uptake by the cover crop and poorly synchronized P release.

Cover crops do not add phosphorus to depleted soils

Cover crops offer some P nutritional
benefits in some circumstances. The variable results (positive and negative
responses) are due to the complicated species, microbial, and environmental
interactions that are not easy to predict. However, it must be remembered that
cover crops do not provide any new P to the soil, but only allow the existing
soil P reserve to be used more efficiently. With removal of P from the field in
harvested products, the nutrient supply must be ultimately replaced with an
additional supply to maintain sustainability.

Mycorrhizal Fungi

Enhanced P uptake is frequently cited as
a primary benefit of mycorrhizal fungi colonization. In this symbiotic
relationship, the plant root provides the energy (carbohydrate) for the fungi
in exchange for improved nutrient uptake and other plant root benefits. Almost
all crop plants form this relationship with mycorrhizal fungi, which is present
in the root zone of most soils.

Sketch of mycorrhizal assocation

Many organic growers encourage the
associations of mycorrhizal fungi with crop roots through the use of cover
crops and rotations. However, frequent tillage commonly used for weed control
causes a disruption of the soil fungal network and may reduce its effectiveness
for providing nutrients to the plant.

The value of mycorrhizal fungi for
supplying P for crops is most apparent in low-P soils. In most cases, plants
growing in soils with medium to high concentrations of P have less mycorrhizal
association than plants in low-P conditions. Therefore, the value of
mycorrhizal fungi is greatest in soils without an adequate supply of P. Similar
to cover crops, mycorrhizal fungi do not provide any additional P to the soil,
but can allow better utilization of the existing soil resource. Commercial
sources of mycorrhizal fungi are available and may be used in specialized

conditions.

Rock Phosphate

Rock phosphate

Rock phosphate (apatite) is a general
term used to describe a variety of globally distributed P-rich minerals. Of the
two main types (sedimentary or igneous), sedimentary rock deposits are the
source of over 80% of the total world production of phosphate rock. Depending
on its geologic origin, rock phosphate has widely varying mineralogy, texture,
and chemical properties. Some rock P is found in hard-rock deposits, while
other rock P is found as soft colloidal (soil-like) material. This great
variation in properties and the accompanying elements present in the rock (such
as carbonate and fluoride) has a large effect on its value as a source of plant
nutrient. This range in properties makes some rock P sources excellent nutrient
sources and other sources quite unsuitable. Unfortunately, the information on P
availability from a specific rock source is not generally available to the
consumer.

The general reaction of rock P
dissolution added to soils to a plant available form is:

Equation
1: Ca5(PO4)3F + 6H+ ↔
5Ca2+ + 3H2PO4– + F–

Note the importance of acidity (H+) and
low Ca2+ in this reaction.

It is difficult to make universally
applicable recommendations for rock P application because so many factors
affect its dissolution and plant availability. However, the key factors to
consider include:

• Soil pH is important in the dissolution
of the rock P (Equation 1). Rock P is much more soluble in acidic soils (soil
pH <5.5). In neutral pH to alkaline soils, rock P typically provides little
benefit for plant nutrition, except under special conditions.

• Particle size influences the
dissolution of rock P by controlling the surface area available for reaction.
However, fine grinding a low-reactivity phosphate rock will not significantly
increase P availability due to its insoluble mineralogical structure.
Conversely, it may not be necessary to finely grind highly reactive rocks used
for direct application to the soil. Many rock P sources are commonly ground to
<100 mesh (0.15 mm) to improve reactivity, but such finely ground material
may be difficult to handle and to spread uniformly.

• Low soil Ca concentrations and high
soil cation exchange capacity favor rock P dissolution since Ca is one of the
reaction products resulting from dissolution. Soil conditions that limit Ca
availability (soil acidity, high leaching, or the presence of organic compounds
that complex exchangeable Ca) also tend to favor rock P dissolution and the
release of P for the plant.

• Other cultural practices that may
improve P availability from rock P include broadcast applications to maximize
soil dissolution reactions, and using management that promotes root
colonization by mycorrhizal fungi. Application of rock P should be made several
weeks or months prior to the anticipated need for plant nutrients. Although
lime applications are important for reducing harmful effects associated with
soil acidity, lime additions tend to reduce the value of rock P as a nutrient
source.

Manure and Composts

Composted dairy manure

These materials are generally excellent
sources of P for plants. Even though these materials are considered as organic
products, over 75% of the total P they contain is present as inorganic
compounds. It is commonly recommended that the P in manure and compost be
considered as 70% available for soils with low soil-test P, but 100% available
for soils testing adequate or high for P.

The ratio of nutrients in composts and
manures does not closely match that required by plants nor in the harvested
products. When manure and compost are used as a primary N source for crops, P
is typically overapplied by 3 to 5 times compared with the crop removal rate.
Long-term use of manures and compost as the primary N
source leads to an accumulation of P in the soil that canbecome an environmental
concern for surface water quality.

Bone Meal

Bone meal, prepared by
grinding animal bones, is one of the earliest P sources used in agriculture.
Most commercially available bone meal is “steamed” to remove any raw animal
tissue. The primary P mineral in bone material is “calcium-deﬁcient
hydroxyapatite” [Ca10–x(HPO4)x(PO4)6–x (OH)2–x (0 < x < 1)], which is
more soluble than rock phosphate, but much less soluble than conventional P
fertilizers. Calcium-deficient hydroxyapatite present in bone meal dissolves:

Similar to rock P, bone meal
is most effective in acidic soils and when the particle size is small. When
used properly, it can be an effective P source. One of the first commercial P
fertilizers was produced by reacting animal bones with sulfuric acid to enhance
the solubility of P.

Concerns have been raised
regarding bovine spongiform encephalopathy (BSE) in cattle and the residual
effect of bone meal as a fertilizer. There are no restrictions on the use of
bone meal and most commercial bone meal products have been heat treated, so the
potential for prion transmission is small.

Guano

Guano as a nutrient source

Guano is most commonly used
as a source of N for plants, but some guano materials are also relatively
enriched in P. Guano is mined from aged deposits of bird or bat excrement in
low rainfall environments. The drying and aging process changes the chemistry
of the P compared with fresh manure. Struvite (magnesium ammonium phosphate)
can be a major P mineral found in guano, dissolving slowly in soil. The limited
supply and high cost of guano generally restricts its use to small-scale
applications.

Summary

There are several options
available for meeting the P requirement for organic production. Growers are
encouraged to first consider locally available materials to meet this need.
Many of the allowed materials are fairly low in nutrient content, therefore
transportation costs may be a concern since relatively large quantities of
amendment may be needed to meet the crop demand. Regular soil and tissue
testing should be conducted by all growers to avoid depletion of soil nutrients
and to prevent inadvertent nutrient accumulation, regardless of production
philosophy and management techniques. BC

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About Me

I am a soil scientist with interest in managing plant nutrients in the best way possible. I am fortunate to be able to work in research and education to be able to accomplish this goal.
After receiving a PhD in Soil Science at the University of California (Riverside), I worked as a Research Scientist for the U.S. government, as a Professor of Soil Science, and now I work for a not-for-profit institution. It's been a wonderful experience!